The aim of this study is to identify risk factors for Alzheimer's disease (AD), dementia, as well as cognitive impairment and cognitive decline, by comparing two populations from very different environments, elderly African Americans from Indianapolis, and the elderly Yoruba from Ibadan, Nigeria. During the current grant periods, our cohorts were enriched by enrolling new participants (Indianapolis: 1,893;Ibadan: 1,942) to add to the survivors of the original cohort. In addition, blood samples were collected (Indianapolis: 1,545;Ibadan: 1,390) for biochemical and genetic analyses. We propose to conduct two incidence studies in the enriched cohorts, two years apart, to address the following Specific Aims: Specific Aim 1: To examine the interaction of APOE4 and lipid levels as risk factors for incident AD and mild cognitive impairment (MCI) in the African Americans and the Yoruba. Specific Aim 2: To examine the relationship between the metabolic syndrome and incident dementia and cognitive impairment no dementia (CIND) in the African Americans and the Yoruba. Specific Aim 3: To examine the relationship between homocysteine levels, levels of folic acid, vitamin B12, and incident dementia and CIND at both sites. Specific Aim 4: To examine the interaction between indices of social involvement and their change over time with the risk of cognitive decline in the African Americans and the Yoruba.